Hello Said... I am working with Aloe species and I concentrate my samples at 35C to avoid degradation of phenolic compounds. However, for your extracts I presume you can use 78C as per the following method: Pg 12 

http://umpir.ump.edu.my/3600/1/CD6404_HAZWANI_ZULKIPLY.pdf

Thank you Dr. Abu-Reidah , Dr. Khan and Darshini. 

I rather prefer to keep temp. at 40 C but after 1 hr. just ethanol has evaporated and water stayed for more than 3 hrs , i tried to adjust the pressure but the water still not evaporating .

I like to share this short communication mentioning that flavonoids and phenolic comps stable up to 120 C 

http://onlinelibrary.wiley.com/doi/10.1002/ejlt.201100240/pdf

Many thankx for your responses 

it should be below 40oC as the heat sensitive compounds get decomposed. But remember removal of MeOH is not easy, you have to have a chiller to cool water circulate in the condenser and need to used good vacuum system.

Most of the saponin and flavanoids are thermostable at working temperature. So, try to validate your temperature at which methanol evaporate from your extracts. I think below 50 it is possible, as it is closed system.

Stay below 40°C. For tannins extraction and purification I use 35-38°C.

You can associate to the rotavapor a vacuum controller in order to reduce the pressure (near vacum condition) (the solvent evaporate faster ).

Hi Said, I also think you should stay below 50C. and use a rotavapor with good vacuum condition. Now if you still getting trouble getting the H2O off, you can consider lyophilization( freeze drying), I think that should get you your powder faster.

I agree with Kennedy's answer, if removal of water is difficult, then a freeze drying can be considered. But a rotavapor with a good vacuum system and a chiller, water can be removed.

Thank you Kennedy, priyani, Neeraj,

But because the aqueous solvent yeild from rotavapor will be large , about 4.5 liters, can i use freeze-dryer in this case? 

and for how long? 

It's depend on your freeze-dryer capacity. I put the sample into a large chemistry beaker cover with tissue paper. If you have a huge layer of aqueous solution the freeze-dry process will be sooo long. I put max. 1 cm of solution in a very large surface and I leave on the freeze-dry 2 days. But really depend on your freeze-dryer. Some freeze-dryer have the possibility to monitoring the process (in this case you can follow and be sure!!!)

Best

In addition to the answers given, you can concentrate your extract by means of solid phase extraction (SPE), so you can reduce solvent volume and change solvent composition. You can try rotaevaporate the ethanol (this is easy with good vacuum, below 40°C), and pass the aqueous solution through an SPE column or catridge and then elute your compounds. See "Solid-phase extraction of phenols" in J Chromatogr A. 2000 Jul 14;885(1-2):291-304 and "Optimization of extraction method for total saponins from Codonopsis lanceolata" in Asian Journal of

Traditional Medicines, 2012, 7(1), where they use C-18 for saponins extraction and methanol for elution.  Best regards

You aré talking about rotatory evaporator? Isn't it? as Sebatian indicate the temperature has to be below 40  C to avoid the decomposition of the unstable compounds. I suppose you are using methanol extract or other solvent that has low boiling point.